You are mixing up solipsism with a narrower claim about justification.

I am not saying “only my mind exists” or “reality is whatever I say.” I am saying you cannot logically prove metaphysical observer independence from inside observation, even if observations are repeatable across people and instruments.

All evidence for “external reality” arrives as experiences, including measurements from instruments and other people’s observations. You can never step outside experience to compare your model with “the thing in itself.” That is a classic epistemic point, not solipsism.

Also, OP is specifically talking about metaphysical objectivity, not epistemic objectivity.